A Spread spectrum communications system is of using spread spectrum modulation technology where spreading sequences are modulated by transmission data. Due to this spread spectrum modulation, a data spectrum with a relatively narrow band width is spread to a wide frequency band and then the spectrum spread signals are transmitted. And in a region (cell or sector) where a base station (BS) provides communications services, there are plural user stations. Such a communications system is excellent in that a low transmission power per unit frequency is consumed, disturbance to other communications can be kept at a relatively low level, and the system has inherently strong capability resistant to ambient noise (external noise like CW noise) admixed in the transmissions process, and to inter-station-interference-noise incoming from interfering mobile stations other than a desired station. Generally this interference-noise includes delayed waves incoming via multipaths from the interfering-stations. However, because communications being performed by numerous stations share the same band, there is a problem such that increasing the number of users to be accommodated becomes difficult, due to the disturbance made by the inter-station-interference-noise. That is to say, it means that so many users can not be accommodated that a frequency-bandwidth to be occupied increases due to spread spectrum modulation, and it results in a decrease in frequency-utilization-efficiency or an increase in power-bandwidth required for 1 bit transmission.
FIG. 14 is a block diagram illustrating the general construction of a mobile communications system which performs direct-sequence spread spectrum communications (DS-SS) via a radio communications channel. Here, a transmitter EX modulates a spreading sequence generated by a sequence generator 1, by multiplying the sequence by a binary transmission data b, thus producing a baseband transmission output s(t). Transmitter TX further modulates this baseband transmission output s(t) with a carrier-wave having a frequency f0 which has been generated by an oscillator 2, thus the spectrum of the signal conveying the data b may spread. Transmitter TX then transmits the resultant output to a radio communications channel. Furthermore, pseudo-noise (PN) sequences each of which is different from one for another user are used as such signatures that the spreading sequences may have a role of discriminating respective users' addresses. As PN sequences, maximum-length code (M) sequences or Gold sequences are generally used.
A receiver RX sends a spread-spectrum-modulated signal to an amplifier 3 via an antenna (not shown in the figures), amplifies the signal up to a required power level, and then frequency-mixes the amplified signal with a local signal fL=(=f0) which a local oscillator 4 has provided. Receiver RX then produces a low frequency component of the resultantly mixed output-signal as a baseband received demodulated signal r(t), by passing it through a low pass filter 5. In this case, coherent demodulation is generally performed.
Baseband demodulated signal r(t) is inputted to a multiplier 7 with the same spreading sequence generated by a sequence generator 6 as the sequence used by above-described transmitter TX. The resultant multiplied output is then integrated by an integrator 8 over the period of the spreading sequence length (1 symbol-frame), to obtain a matched filter output. A received binary data {circumflex over (b)} is detected by making this output on the hard decision. This function compares it with a threshold level by a detector 9 at the end of the above-described frame period (correlative demodulation). A control signal made based upon this detected data is inputted into the control terminal of sequence generator 6 via a synchronization detector 10, and the generating timing of the spreading sequence is controlled so that the sequence phase may be synchronized with that of the received signal. Furthermore, in receiver RX shown in FIG. 14, the arrangement of the multiplying functions provided by local oscillator 4 and sequence generator 6 is often exchanged each other; however, the overall demodulation function remains the same.
FIG. 15 schematically shows spectra of a received signal on a demodulation process. In FIG. 15(a), reference numerals 11 and 12 denote a spectrum of a spread spectrum modulated signal which is an input signal to a receiver, and a spectrum of admixed environmental noise, respectively. When the receiver demodulates (dispreads) this signal with the spreading sequence, spread spectrum modulated signal 11 which has been spread over a wide frequency-bandwidth as shown in FIG. 15(b), is converted into a narrow-band-signal 13, and environmental noise 12 is converted into a signal 14 whose spectrum is distributes over a wide frequency-band. Accordingly, this communications system can suppress the disturbance due to environmental noise.
FIG. 16 is a diagram showing the waveform of a baseband transmitting symbol-frame (hereafter the description of symbol is omitted) in a conventional direct-sequence spread spectrum communications system (DS-SS). In the figure, n, T, Tc, bn, g(i), s(i) and s(t) denote the frame number which shows a time sequence order of symbol-frames, a transmitting symbol-frame period for a data, the chip-period, the n-th binary transmit-information, the spreading (impulse) sequence, the transmit-frame in discrete representation, and the transmit-frame in continuous time representation, respectively. Here is shown a sequence with length L=7 (chips) as an example of g(i). The n-th binary data bn takes 1 and −1 corresponding to the data 1 and 0, respectively. Transmit-frame s(t) is a transmission-baseband-waveform obtained by replacing the respective impulses in s(i)=bng(i) with square waves. Thus, discrete sequence g(i) and its continuous time-waveform g(t) are given by:
                              g          ⁡                      (            i            )                          =                                            ∑                              i                =                0                                            L                -                1                                      ⁢                                                  ⁢                                          c                i                            ⁢                              δ                ⁡                                  (                                      t                    -                                          i                      ⁢                                                                                          ⁢                                              T                        c                                                                              )                                                              =                                                    (                                                      c                    0                                    ,                                      c                    1                                    ,                                      c                    2                                    ,                                                                          ⁢                                      …                    ⁢                                                                                  ⁢                                          c                                              L                        -                        1                                                                                            )                            ⁢                                                          ⁢                              c                i                                      ∈                          ±              1                                                          (        1        )                                          g          ⁡                      (            t            )                          =                                            ∑                              i                =                0                                            L                -                1                                      ⁢                                                  ⁢                                          c                i                            ⁢                                                q                  1                                ⁡                                  (                                      t                    -                                          i                      ⁢                                                                                          ⁢                                              T                        c                                                                              )                                            ⁢                                                          ⁢              0                                ≤          t          ≤          T                                    (        2        )                                                                                                      q                  ⁡                                      (                    t                    )                                                  =                1                                                                                                        t                                                  ≤                                                      T                    c                                    2                                                                                                                                                          ⁢                                  =                  0                                                                                                                        t                                                  ≤                                                      T                    c                                    2                                                                    }                            (        3        )            where ci(i=0,1,2, . . . L−1), δ, and q(t) are the i-th chip amplitude of a spreading sequence, the delta function, a square-wave-function, respectively. An actual radio-band transmit-wave sa(t) is generally produced by modulating a carrier-wave by baseband transmitting waveform s(t)[g(t) or −g(t)] having a bandwidth which is limited less than Te=Te−1.
Furthermore, when a square waveform is used as q(t), inter-symbol-interference takes place. To avoid this phenomenon, for Eq. (3) such a waveform is used that the auto-correlation function may take zero at the adjacent sampling points (called an f-type-sampling-function whose DFT conversion has cosine roll-off characteristics). In this case, if a receiver prepares the same waveform q(t) as that of the transmitting side, and performs correlative demodulation by multiplying a received symbol-frame by q(t) and a local carrier-wave. As a consequence, the desired wave components of the received signal will be obtained as an impulse sequence indicated in Eq. (1), where it is not subjected to the interference due to preceding or succeeding chips. That this impulse sequence is dispread with spreading sequence g(i) and then integrated. The n-th transmitted information bn can be detected, by making the integrated output on the hard decision. If an f-type-sampling-function with a roll-off factor of α (generally taking a value between 0.1 and 1) is used, the radio bandwidth BS of the above-described spreading signal and the radio bandwidth BD of the transmit-data is given by:BS=(1+α)/TCBD=(1+α)/(LTC)=BS/L.  (4)However, L>>1 is required in a practical system, and in spite of using a bandwidth L times larger than that of one user, for the maximum number K of simultaneous calls (the number of above-described user-station), K<<L is required. Therefore the simultaneous transmission capacity/Hz is (K/L) times as large as that of a time-division-multiple-access radio system (TDMA). Consequently, there is such disadvantage that existing CDMA systems have frequency utilization-efficiency not higher than that of time-division-multiple-access system.
Thus, the reason why the number K of user stations compared to the sequence length cannot be set to a large value is that the cross-correlation-coefficient between sequence g0(t) assigned to a desired station and a different sequence gk(t)(k≠0) assigned to the other mobile station cannot be sufficiently small. Furthermore, considering delayed waves due to multipath, the cross-correlation-coefficient between many delayed sequence groups coming from respective user stations and sequence g0(i) increases. That is, that the code-error-rate considerably deteriorates, because many interference waves with high cross-correlation come in, and therefore it is impossible to increase the number of user stations K and to raise the frequency-utilization-efficiency.
To reduce the disturbance due to the above-described interference-noise, many methods have been studied so far, the sufficient noise suppression effect has not been obtained. Here let's explain 4 kinds of preceding techniques having close relationship with this invention.
(A) Selective transmission system with parallel combinatorial sequences [Reference: S. Sasaki, H. Kikuchi, J. Zhu, and G. Marubayashi, “Error Rate Analysis of Coherent and Differential Multiphase Parallel Combinatorial Spread Spectrum System,” IEICE Trans. Fundamentals, Vol.E80-A, No.7, pp.1196–1203, 1997-07.]
(B) Isolated pilot assisted analytical demodulation system [Reference: Mitsuhiro Tomita, Noriyoshi Kuroyanagi, Naoki Suehiro, Shinya Matsufuji, “A Pilot Frame Assisted CDMA System using Gold Sequences”, Proceedings of WPMC'99, 9.4, pp.346–353, Amsterdam, 1999-09]
(C) Rake reception system [Publication: Ramjee Prasad “CDMA for wireless personal communications”, Artech House 1996, Sections 11.5 and 11.6 (p348, pp365–366)]
(D) Space-time coding system [Reference: Ben Lu and Xiaodong Wang, “Iterative Receivers for Multiuser Space-Time Coding Systems”, IEEE JSAC, VOL.18. No.11, November 2000]
In system (A), a transmitter prepares a sequence-set consisting of M kinds of basic spreading sequences, selects γ sequences in it, multiplies them by a set of binary information, and adds up all the resultant sequences to transmit. In this case, because a binary information-set of I bits to be transmitted corresponds to combinations which are determined by M and γ, the information quantity per symbol-frame is given by:I=γ+log2(MCγ)  (5)Consequently, this system is an example of Multi-ary systems and the improvement in frequency-utilization-efficiency has been expected.
When a transmit-symbol-frame arrives at a receiver, generally it turns out to be (J+1) waves consisting of the main wave and J delayed waves that are caused by multipaths. If the number of users is assumed to be K, the receiver receives (γ−1) waves (sequences) from the desired station as a self-interference-component, and γ(K−1)(J+1) waves from the other interfering stations as an inter-station-interference-component. Therefore the disturbance due to these interferences is considerably large in general. And this system requires many (MK kinds) spreading sequences, and the average value of cross-correlation-functions between an arbitrary pair of sequences in the sequences increases, as the family size MK increases. That is to say that the interference power increases in proportion to M, K, and J. Consequently, it is impossible to increase M and K to a large extent, in order to obtain desirable error-rate-characteristics by using system (A), as a result, it is impossible to raise the frequency-utilization-efficiency.
In system (B), a transmitter of each user transmits isolated pilot-frames so as not to suffer the disturbance of interference waves due to the other users. A receiver receives these isolated pilot-frames and always prepares highly accurate pilot-response from all the users. Each user transmitter produces a transmit-data-frame using both a sequence which is allocated to the user and a carrier wave which is common for all the users, and the produced frame is transmitted.
That is to say that system (B) is such a system that a base-station BS accurately acquires the transmission-path-characteristics (channel) from each user to BS by means of that each user transmits isolated pilot-signals to BS. Consequently, BS can obtain the pilot-response (channel-response) {psk}(s: the number showing the shift position of the correlation-function for s=0,1,2, . . . L−1), corresponding to the transmission-path from the k(=0,1,2, . . . K−1)-th user. If a transmit-information of the k-th user is bk, the correlation response {Φs} of a received-frame r(t) is given by{Φs}=bk{psk}+Δbk{psk}.  (6)The second term on the right-hand in the above equation is an error component corresponding to a white noise included in r(t). By solving Eq. (6), a detected output {circumflex over (b)}k=bk+Δbk of the transmitted information can be obtained. This system can remove the effect of interference waves almost completely. Because {psk} includes an interference component due to the white noise, perfect separation of interference waves can not be achieved, however, for CDMA systems to which interference waves give stronger disturbance than that white noise does, the advantage which almost all the interference wave components can be removed is valuable.
However, since system (B) uses the correlation responses, {psk} makes a matrix with a size of L×L, as a result, for the parameter k(=0,1,2, . . . K−1), a relation given by K≦L is required. That is to say that the number of users who can be accommodated is limited by the spreading sequence length, and it raises a problem that the frequency-utilization-efficiency can not be sufficiently increased.
System (C) is a demodulating detection method used for a multipath environment where many of delayed waves generated by a desired user's transmitted frame are contained in a received frame. This system has a function such that not only a main wave but also the delayed waves are demodulated by multiple fingers (demodulation circuits operating synchronously with delayed waves) and their outputs are summed up. It is reported that the error increases considerably due to the inter-station-interference-disturbance as the number of users increases, because the above stated finger output includes inter-station-interference-components consisting of main waves and the delayed waves both coming from interfering stations.
System (D) is a CDMA communication system using multiple transmit- and receive-antennae. In this system, respective user transmitters produce plural frame-sequences by applying convolutional coding to a transmit-data-frame-sequence, and then transmit these coded frame-sequences via plural antennae. A base-station receiver obtains demodulated outputs for respective users with a multi-user demodulator, using a signal group received via plural (Nd) antennae, and then produces the first detected value bn on the n-th frame. The time sequence of the first detected value bn has a high error-rate. Based on these plural (Nd) sets of similar time sequences, a convolutional decoder performs decoding to produce the second detected values. By feeding back the detected values to the above-stated multi-user-demodulator and the convolutional decoder, the above-stated demodulation and decoding are performed to obtain the third detected values. This is such a system that the final detected values are decided by repeating the above stated processing multiple times.
System (D) requires a considerably complicated processing and a long delay time. It is the problem of this system that nevertheless it fails to increase the frequency-utilization-efficiency. In contrast, an actual sequence increasing technique of the present invention to be stated later is a system capable of obtaining the above stated first detected value bn with a low error-rate, to acquire an excellent performance by using a simple processing, and it is a quite different technique from system (D).
The present invention has been done to overcome the performance limit in terms of the frequency-utilization-efficiency or power-bandwidth-product, as CDMA system evaluation measure, which the preceding systems have indicated, by providing multiary system design techniques with spreading sequence sets.